A physical quantity such as load or pressure detected by a sensing circuit is converted to a detection signal. In resent years, the detection signal representing the physical quantity is generally converted to digital data by an analog-to-digital (A/D) conversion circuit and the digital data is input to a microcomputer. The microcomputer performs a computation such as temperature compensation so that a detection value corresponding to the physical quantity can be obtained. The sensing apparatus has a temperature sensing circuit for generating a temperature signal having a voltage level corresponding to a temperature of the sensing circuit. The microcomputer performs the temperature compensation based on the temperature signal.
In a well-known A/D conversion circuit, a voltage signal to be converted is compared with a reference voltage by an analog comparator and converted into binary numbers. However, in such an A/D conversion circuit, it is difficult to significantly improve the conversion speed and to allow the binary numbers to reflect a slight change in the voltage signal. Accordingly, the A/D conversion circuit takes a long time to calculate the detection value and accuracy of the calculated detection value is reduced.
An A/D conversion circuit of a sensing apparatus disclosed in U.S. Pat. No. 6,307,496 corresponding to JP-A-H11-44585 overcomes the above problem. The A/D conversion circuit includes a ring-gate-delay circuit (RGD) having multiple inverting circuits connected in a ring pattern. Each inverting circuit performs an inverting operation at a variable speed that depends on a power voltage of the RGD. In the A/D conversion circuit, while a voltage signal to be converted to digital data is applied to the RGD as the power voltage of the RGD, a pulse signal is input to the RGD. The voltage signal is converted to digital data based on the number of times the pulse signal circulates through the RGD. In the A/D conversion circuit, the conversion speed and resolution can be significantly improved.
However, the inverting operation speed of the inverting circuits depends on not only the power voltage of the RGD but also a temperature of the RGD. Therefore, the A/D conversion circuit may produce an output value including a nonlinear factor due to the temperature dependence on the RGD. Consequently, accuracy of the output value may be reduced.
The A/D conversion circuit includes a reference voltage generation circuit for generating a reference signal having a voltage level that remains constant regardless of load (physical quantity) applied to the sensing circuit and temperature of the sensing circuit. The A/D conversion circuit converts the reference signal to a reference data. By using the reference data, the nonlinear factor due to the temperature dependence on the RGD can be eliminated.
In practice, however, there are offsets between the detection signal, the temperature signal, and the reference signal due to differences in electrical characteristics between the sensing circuit, the temperature sensing circuit, and the reference voltage generation circuit. The A/D conversion circuit includes a memory for storing voltage correction data corresponding to the offsets. When being applied to the RGD as the power voltage, the detection signal, the temperature signal, and the reference signal are corrected by the voltage correction data, respectively. Therefore, the digital data converted by the A/D conversion circuit includes no error factor due to the offsets. In the sensing apparatus disclosed in U.S. Pat. No. 6,307,496, therefore, the accuracy of the output value corresponding to the physical quantity can be improved.
In the sensing apparatus, temperature dependences (offset and gain) of the detection value on both the sensing circuit and the A/D conversion circuit are corrected. Accordingly, an equation for calculating the output value includes a correction factor determined by the temperature dependences. In order to determine the correction factor, the sensing apparatus should be tested at two or more temperatures (e.g. −40° C. and 120° C.) at a point where the sensing apparatus indicates zero and at a point where the sensing apparatus indicates a maximum value.
In the calibration test for correcting for the temperature dependences, a desired load needs to be applied to the sensing apparatus at high and low temperatures. Therefore, costly large-scale equipment is required for the calibration test to be performed.
Further, the sensing apparatus disclosed in U.S. Pat. No. 6,307,496 needs a temperature sensing circuit, and accordingly manufacturing cost of the sensing apparatus may be increased.
In the sensing apparatus, the correction factor included in the equation for calculating the detection value depends on not only the sensing circuit but also the A/D conversion circuit. Therefore, whenever the A/D conversion circuit is used, the calibration test is required.